SIDEROBLASTIC ANEMIAS

INTRODUCTION

The sideroblastic anemias are a heterogeneous group of disorders with two
common features: ring sideroblasts in the bone marrow (abnormal normoblasts
with excessive accumulation of iron in the mitochondria) and impaired heme
biosynthesis (Bottomley, 1982; May et al, 1994). The etiology, epidemiology,
pathophysiology and treatment of these conditions differ vastly. The mitochondrion
is the nexus of sideroblastic anemia, however. Disturbed mitochondrial
metabolism is at the center of all sideroblastic anemias in which a cause
has been determined.

Figure 1.Schematic Representation of Heme Biosynthesis

Heme biosynethsis begins in the mitochondrion with the
formation of 5-aminolevulinic acid. This molecule moves to the cytosol
where a number of additional enzymatic transformations produce coproporphyrinogin
III. The latter enters the mitochondrion where a final enzymatic conversion
produces protophorphyrin IX. Ferrochelase inserts iron into the protophorin
IX ring to produce heme.

Mitochondria are the source of the oxidative phosphorylation that provides
most of the ATP used by eukaryotic cells. The mature erythrocyte is the
sole mammalian cell that lacks mitochondria and relies totally on glycolysis
as an energy source. Most cells contain between 100 and 300 mitochondria
(Jaussi, 1995). Mitochondria are semi-autonomous organelles that likely
began as freestanding prokaryotes that invaded eukaryotic cells more than
a billion years ago (Jansen, 2000). A symbiotic relationship eventually
developed between these prokaryotic cells and their eukaryotic hosts. The
former prokaryotes lost the capacity for independent existence but became
indispensible to the eukaryotic cells.

Mitochondria retain vestiges of their former independent life. Most
importantly mitochondria have small DNA genomes (about 16 kb) and replicate
independently within their host cells. Mitochondrial DNA retains many features
of prokaryotic genomes, including a circular structure lacking introns
(Boore, 1999). The mitochondrial genome encodes a small number of proteins
as well as several transfer RNA molecules. Mitochondrial DNA lacks chromatin
and the organelles have a limited capacity for DNA repair. These characteristics
mean that mutations in the mitochondrial genome that produce sideroblastic
anemia likely will remain uncorrected.

Mitochondria replicate independently of the nuclear genome (Kuroiwa,
2000). When cells undergo mitosis, mitochondria distribute stocastically
to the daughter cells. Acquired mitochondrial defects therefore pass unevenly
to the daughter cells. This property is important to the some of the hereditary
mitochondrial disorders that produce sideroblastic anemia. This characteristic
also poses a conundrum with respect to the acquired sideroblastic anemias.
A few cases of sideroblastic anemia associated with myelodysplasia have
acquired mutations that impair function of some cytochromes (see below).
The mutation presumably began as an alteration in a single mitochondrion.
The puzzling question is how mitochondria with impaired enzymatic function
come to predominate in the cells. Each mitochondrion has several genomes
(i.e., several circular DNA molecules), and each cell has several hundred
mitochondria. Logically, the defective mitochondrion should be at a survival
disadvantage. The acquired sideroblastic anemias remind us that much remains
to be learned about the physiology of these fascinating organelles.

Figure 1 is a simplified schema of heme biosynthesis. The process begins
in the mitochondrion with the condensation of glycine and succinyl-CoA
to form delta-amino levulinic acid (ALA) (Bottomley et al, 1988).
Pyridoxal phosphate is a cofactor in the reaction. ALA then moves to the
cytoplasm where several additional enzymatic transformations produce coproporphyrinogen
III. This molecule enters the mitochondrion where additional modifications,
including the insertion of iron into the protoporphyrin IX ring by ferrochelatase,
produce heme. Defects in the cytoplasmic steps of heme biosynthesis cause
various forms of porphyria. Functional abnormalities of the enzyme porphobilinogen
deaminase, for instance, produce acute intermittent porphyria (Mustajoki
et
al, 2000).

In contrast, defects in the steps of heme biosynthesis that occur within
the mitochondrion produce sideroblastic anemias. For instance, perturbations
of the enzymatic activity of delta-amino levulinic acid synthase (ALAS)
produce sideroblastic anemia (Bottomley et al, 1992). The X-linked
hereditary sideroblastic anemias result from mutations in the gene encoding
ALA-S. Isoniazide, a drug used to treat Mycobacterium tuberculosis
infection and that commonly produces sideroblastic anemia in people who
fail to use pyridoxine prophylaxis, directly impairs ALAS function (Yunis
et
al, 1980). A group of disorders termed "mitochondrial cytopathies"
result from deletions of portions of the mitochondrial genome (Buemi et
al, 1997). The consequent profound mitochondrial dysfunction produces
sideroblastic anemia in some of these disorders. The best known of these
is Pearsonís syndrome, which was the first characterized entity in this
class of disease (Pearson et al, 1979).

Most commonly, the sideroblastic anemias are classified as hereditary
or acquired conditions (Table 1). Most of the hereditary forms are X-linked,
although some family studies have revealed autosomal dominant or autosomal
recessive modes of transmission (Cox et al, 1990). In many instances,
isolated cases of congenital sideroblastic anemia defy classification as
they lack the well-documented pedigrees needed to firmly establish modes
of transmission (Dolan et al, 1991).

Table 1. Classification of Sideroblastic Anemias

Category

Groups

Etiology

Hereditary

Congenital X-linked

ALAS-2 mutations
Xp11.12 linked disorders

Autosomal dominant

Unknown

Autosomal recessive

Unknown

Mitochondrial Cytopathy

mtDNA
Chromosome 4p16 abnormality

Acquired

Myelodysplasia

mtDNA point mutations, and unknown

Drugs

Ethanol, INH, chloramphenicol, cycloserine

Toxins

Lead, zinc

Nutritional

Pyridoxine deficiency, copper deficiency

The frequency of the acquired sideroblastic anemias far exceeds that
of the hereditary varieties. Drugs and toxins lead this category, propelled
largely by the high frequency of alcohol abuse in many societies (Larkin
et
al, 1984). Alcohol produces sideroblastic anemia in only a small fraction
of people. A large number results, however, when this small fraction is
factored against the very large number of alcohol abusers. The next largest
subgroup is itself a subset of the myelodysplastic syndromes (MDS) (Hast,
1986). Rarely, nutritional imbalances or other insults (e.g. hypothermia)
produce sideroblastic anemia (O'Brien et al, 1982).

The exact mechanism by which disturbed heme metabolism produces sideroblastic
anemias remains elusive. Heme is an essential component of many mitochondrial
enzymes (cytochromes b, c1, c, a, a3)
as well as cytosolic enzymes such as catalase. The molecule is also an
integral component of hemoglobin where it has both structural and functional
roles. Heme modulates translation of globin mRNA, stabilizes the globin
protein chains, and mediates reversible oxygen binding.

ALAS is both the first and the rate-limiting enzyme in heme biosynthesis
(Bottomley et al, 1988). Heme modulates its activity through feedback
inhibition. The two genes that encode ALAS have been cloned and assigned
chromosomal locations. The gene that encodes ALAS-1 (also called ALAS-n)
exists on chromosome 3 (3p21) (Bishop et al, 1990). This ubiquitous
enzyme is particularly abundant in the liver. ALAS-1 provides the basal
heme production needed by all cells and maintains a relatively stable level.

The enzyme directly relevant to sideroblastic anemia is ALAS-2 or ALAS-e.
A gene on the X chromosome (Xp11.21) encodes this enzyme, whose expression
is restricted to the erythroid lineage (Cotter et al, 1992a). In
addition to heme-mediated feedback inhibition of enzymatic function, ALAS-2
is a member of a small family of genes whose expression is modulated by
iron (Cox et al, 1991).

The best-characterized genes of this family are those encoding ferritin
and the transferrin receptor (Klausner et al, 1989). Ferritin mRNA
contains a conserved stem-loop sequence in the 5í-untranslated region called
the iron response element (IRE). A homologous sequence exists in the 5í-untranslated
region of the ALAS-2 message. In contrast, transferrin receptor mRNA has
five IRE elements all in the 3í-untranslated region.

Two cytoplasmic proteins called the iron regulatory proteins 1 and 2
(IRP1 and IRP2) bind to the IRE regions of the messenger RNA. In the absence
of iron, IRP1 binds to the IRE elements in the 5í-untranslated regions
of the messages encoding ferritin and ALAS-2. The IRE/IRP complex blocks
message translation, lowering the biosynthesis of ferritin and ALA-2. This
response makes teleological sense. In the absence of iron, the cell does
not need the iron storage protein, ferritin. Similarly, in the absence
of iron, erythroid precursors dampen production of ALAS-2, with a consequent
diminution in protoporphyrin IX synthesis. Without iron, the cells need
not produce protoporphyrin IX since conversion of this molecule to heme
is impossible.

A defect in the ALAS-2 IRE theoretically could produce a sideroblastic
anemia. A mutation that makes the IRE a high affinity target for the IRP
proteins irrespective of ambient iron concentrations, for instance, would
substantially lower the level of ALAS-2 in the cells. No such defect has
been described, however. This contrasts with ferritin, where abnormal IRE
elements in the message produce a familial syndrome of serum hyperferritinemia
and associated cataracts (Beaumont et al, 1995). Since ALAS is an
enzyme, extreme defects in its production might be required to produce
clinical syndromes. Changes in enzymatic activity might compensate for
lesser perturbations in enzyme level.

The signal feature of sideroblastic anemia is mitochondrial iron deposition
(Koc et al, 1998). High resolution microscopy of cells stained with
Perlís Prussion blue usually demonstrates one to four siderocytes throughout
the cytoplasm of normoblasts. Large clusters of siderocyte bodies encircle
the nucleus of ring sideroblasts (Figure 2). Ring sideroblasts usually
constitute at least 15% of the normoblasts, and the figure often reaches
50%.

Figure 2. Ring Sideroblasts

The bone marrow aspirate from a patient with sideroblastic
anemia in this photomicrograph was stained with Perl's Prussian blue. The
arrow indicates a normoblast with a greenish halo of material stained by
Perl's Prussian blue surrounding the nucleus. Electron microscopic examination
would should these to be iron-laden mitochondria.

Electron microscopy shows crystalline iron deposits in the cristae of
the mitochondria (Grasso et al, 1969). The basis of this phenomenon
is unknown. Simple cellular iron overload is not the answer. Massive cellular
iron overload occurs in both hereditary and transfusional hemeochromatosis.
Iron laden mitochondria are manifest in neither disorder. Sideroblasts
show normal iron uptake, but subsequent poor incorporation into heme (May
et
al, 1982). Mishandling of iron by mitochondria could be the basis of
the iron deposits. Our limited understanding of mitochondrial iron metabolism
has precluded testable hypotheses, however.

Mitochondrial iron deposits might be more than a morphological curiosity.
Iron catalyzes the formation of reactive oxygen species through Fenton
chemistry (Liochev et al, 1994). Molecules such as the hydroxyl
radical (ïOH) arise in settings where oxidation reactions occur in proximity
to iron (Gutteridge et al, 1991). The oxidative metabolic machinery
of the mitochondrion makes it an ideal site for the generation of reactive
oxygen species. The primary damage that produces iron-laden mitochondria
in sideroblastic anemia could produce a feedback loop of escalating mitochondrial
injury. The hydroxyl radical, for instance, promotes lipid and protein
peroxidation as well as cross-links in DNA strands. The latter phenomenon
could be particularly injurious given the dearth of DNA repair enzymes
in mitochondria.

HEREDITARY SIDEROBLASTIC ANEMIAS

X-linked sideroblastic anemia

In 1945, Thomas Cooley described the first cases of X-linked sideroblastic
anemia in two brothers from a large family in which the inheritance of
the disease was documented through six generations (Cooley, 1945). Although
rare, the disorder nonetheless is the most common of the hereditary sideroblastic
anemias. The more frequent of the two known genetic defects involves the
ALAS-2 gene. The second consists of abnormalities in the Xq13 region of
the X chromosome (Raskind et al, 1991). Although no specific gene
mutation has been identified for the latter, several laboratories have
demonstrated a linkage with the phosphoglycerate kinase gene (PGK), which
itself is near the gene for Menkes syndrome (an inborn abnormality of copper
transport). Copper is essential to cellular iron uptake from transferrin.
In addition, cooper is a cofactor for several enzymes of the mitochondrial
electron transport chain. Abnormalities in the Xq13 chromosome region could
subtly perturb cellular copper metabolism. Sideroblastic anemias in this
case would be a secondary phenomenon in a condition whose primary disturbance
involves copper metabolism.

Missense mutations of the ALAS-2 gene produce most cases of X-linked
sideroblastic anemia (Cotter et al, 1992b; Bottomley et al,
1992; Cox et al, 1992; Edgar et al, 1997; Cox et al;
1994). Years after their initial evaluation, investigators located several
members of the pedigree originally described by Cooley and analyzed their
DNA using current techniques in molecular biology (Cotter et al,
1994). These now adults indeed had missense mutations involving the ALAS-2
gene. Rarely has anyone correctly described two major disorders that withstood
the rigors of subsequent scientific investigation by more powerful analytical
tools. The other disorder in this instance is, of course, Cooley's anemia,
now known as ß-thalassemia major (Cooley et al, 1927).

The mutations of the ALAS-2 gene can be classified according to their
effects on the enzyme product: low affinity for pyridoxal phosphate, structural
instability, abnormal catalytic site, or increased susceptibility to mitochondrial
proteases. Any of these abnormalities decrease the biosynthesis and/or
activity of ALAS and consequently lower heme production. The net result
is low hemoglobin production by the developing normoblasts and anemia.
Ineffective erythropoiesis results from the imbalance between heme biosynthesis
and globin chain production.

Hereditary X-linked sideroblastic anemia usually occurs in males, of
course. Cases involving females in a pedigree derive most often from skewed
lyonization patterns in the affected girls (Seip et al, 1971; Dolan
et
al, 1991; Seto et al, 1982; Buchanan et al, 1980). Bottomley
and colleagues presented an abstract describing three unrelated families
in which females had clinical features of hereditary sideroblastic anemia.
Curiously, no affected male existed in the pedigrees. DNA sequencing confirmed
ALAS-2 gene mutations on the X chromosome. Skewed lyonization explained
the clinical symptoms in the probands. In one pedigree, the mother and
one sister of the proband were unaffected carriers because they did not
have skewed lyonization patterns. The absence of affected males suggested
to the investigators that the hemizygous state for the mutations produced
an embryonic lethal situation. No follow-up report to this intriguing observation
exists.

Other hereditary forms of sideroblastic anemia

Reports exist of both autosomal dominant and autosomal recessive modes
of transmission of hereditary sideroblastic anemia (Kasturi et al,
1982; Kardos et al, 1986; van Waveren et al, 1987). The genes
involved in these cases remain elusive. Some investigators postulate that
the products of the affected genes somehow dampen the biosynthesis or the
activity of ALAS with a consequent diminution of heme production. ALAS
is synthesized on cytoplasmic ribosomes as a 65 kDa proenzyme whose leader
mediates entry into the mitochondrion (Ferreira et al, 1995). The
pro-sequence is clipped, producing a 59.5 kDa active enzyme. A clip of
the pro-sequence in the cytoplasm would produce enzymatically active ALAS
that could not enter the mitochondria.

Jardine and colleagues described congenital sideroblastic anemia in
a brother and a sister born to unaffected parents, making likely autosomal
recessive transmission (Jardine et al, 1994). The ALAS-2 enzymatic
activity in the erythroblast fraction of the marrow was normal. Genetic
analysis excluded ALAS-2 gene mutations as the cause of the sideroblastic
anemia. The investigators hypothesized that an autosomal gene regulates
ALAS-2 gene expression. A defect in this ALAS-2 regulatory gene would produce
sideroblastic anemia as a downstream event. The fact that ALAS-2 enzymatic
activity was normal in the erythroblast fraction, however, means ALAS-2
gene expression was not the problem. A defect that prevented ALAS-2 localization
to the mitochondria would produce sideroblastic anemia. ALAS-2 production
and enzymatic activity would be normal, as the investigators found. Sideroblastic
anemia would result from a deficiency of ALAS-2 in the mitochondria. Unfortunately,
the investigators did not examine the subcellular localization of ALAS-2.

Mitochondrial Cytopathies

The mitochondrial cytopathies are a heterogeneous group of disorders produced
by deletions in the mitochondrial genome (Egger et al, 1981; Kitano
et
al, 1986; Runge et al, 1986). Some of the disorders include
deletions of up to 30% of the 16 kb mitochondrial genome. Two factors contribute
to the peculiar inheritance patterns in these disorders. First, the independent
mitochondrial replication combined with random segregation into the daughter
cells at mitosis means that by pure chance newly replicated cells can have
more or fewer defective mitochondria. Second, mothers alone transmit mitochondria
to offspring. Mitochondrial cytopathies therefore are maternally transmitted
with the degree of expression determined by the stochastic segregation
of mitochondria into ova at meiosis. A mother with mild manifestations
of a syndrome can have one child who is unaffected and another who has
extremely severe disease (mitochondrial heteroplasmy).

In 1979, Pearson and colleagues described children from several unrelated
families who manifested sideroblastic anemia and exocrine pancreatic dysfunction
(Pearson et al, 1979). Subsequent cases of what is now called Pearsonís
syndrome also had varying degrees of lactic acidosis, hepatic and renal
failure. Bone marrow examination showed in addition to the ring sideroblasts,
large vacuoles in the erythroid and myeloid precursors. Few of the probands
survived past early childhood.

The disorder results from mitochondrial DNA deletions that often are
as large as 4 kb (Cormier et al, 1990). Southern blots of mitochondrial
DNA show genomes of normal size along with the truncated DNA. Variation
in the intensity of the two bands reflects mitochondrial heteroplasmy in
the mother and offspring (Bernes et al, 1993). These deletions impair
the biosynthesis of various components of the mitochondrial respiratory
chain critical to mitochondrial function. Other disorders result from deletions
of different portions of the mitochondrial genome (e.g., myopathy, encephalopathy,
ragged red fibers [in muscles] and lactic acidosis, or MERRL). Sideroblastic
anemia is not part of the clinical spectrum of these syndromes.

An instructive form of sideroblastic anemia occurs in patients with
Wolframís syndrome (DIDMOAD; diabetes insipitus, diabetes mellitus, optic
atrophy, and deafness) (Borgna-Pignatti et al, 1989). The condition
results from large deletions of the mitochondrial genome. The heteroplasmic
nature of the mitochondrial defect in Wolframís syndrome is typical of
a mitochondrial cytopathy (Rotig et al, 1993). The anemia in affected
individuals results from both sideroblastic and megaloblastic derangements
in erythroid maturation.

Unlike Pearsonís syndrome and other mitochondrial cytopathies, Wolframís
syndrome shows an autosomal recessive pattern of inheritance. Family studies
point to a nuclear gene on chromosome 4 (4p16) as the cause of Wolframís
syndrome (Barrientos et al, 1996). The current hypothesis holds
that the gene on chromosome 4 contributes somehow to the stability of the
mitochondrial genome. Without the function of this still unidentified gene,
mitochondrial DNA falls victim to damage that ultimately produces large
deletions. Impairment of energy production and other mitochondrial functions
produce the sideroblastic/megaloblastic anemia, along with the other ultimately
deadly defects in this disorder.

ACQUIRED SIDEROBLASTIC ANEMIAS

Acquired sideroblastic anemias are much more frequent than the hereditary
forms. The defect sometimes surfaces in the context of a myelodysplastic
syndrome. Other instances of acquired sideroblastic anemias reflect exposure
to toxins or deficiencies of nutritional factors. The hereditary sideroblastic
anemias nearly always manifest in childhood or infancy. In contrast, the
acquired forms, particularly those associated with myelodysplasia, nearly
always occur in older adults.

Myelodysplastic syndromes

The myelodysplasias are a group of disorders whose common feature is hematopoietic
stem cell dysfunction (Basa, 1992). In some instances, chromosomal abnormalities,
such as the 5q- anomaly, accompany the condition. Dysplastic
features occur in all three hematopoietic cell lines. Common defects include
prominent nucleoli, abnormal granulation of myeloid precursors, multinucleated
erythroid precursors, and small megakaryocytes that often contain a single
nucleus. Ineffective erythropoiesis is common. Marrow function deteriorates
over time, manifested as peripheral blood cytopenias of the three hematopoietic
cell lines. About 15% of patients with myelodysplasia develop acute leukemia.

One subset of patients has dysplastic features confined to the erythroid
series. Chromosomal abnormalities occur in some people, but usually are
relatively selective with defects such as the 5q-
anomaly. This condition has been named "pure sideroblastic anemia" (PSA).
In the absence of myeloid or platelet abnormalities, anemia dominates the
clinical course. The need for frequent transfusion produces iron
overload that can impair cardiac function and injure the liver. With
adequate chelation therapy,
these patients can survive and even thrive for many years. Most importantly,
very few patients develop acute leukemia.

The second group of patients has abnormalities in all three cell lines
in addition to ring sideroblasts. Chromosomal abnormalities are prominent
and often include multiple deletions, trisomy, or inversions. Although
the anemia is troublesome, neutrophil and platelet abnormalities are the
dominant problems for these patients. Infection is the most common cause
of death, due both to neutropenia and neutrophil dysfunction. Bleeding
is a common problem due to thrombocytopenia and/or platelet dysfunction.
As many as 15% of patients who survive these problems develop an acute
leukemia that often is refractory to treatment. The prognostic implications
of these two forms of sideroblastic anemia associated with myelodysplasia
make mandatory detailed morphological and cytogenetic evaluation at the
time of diagnosis.

The ring sideroblasts associated with myelodysplastic syndromes manifest
in both the early and late erythroid precursors. This contrasts with the
hereditary X-linked conditions in which prominent sideroblastic rings generally
appear only in the more differentiated normoblasts. Only recently, have
investigators pinpointed some of the abnormalities that might explain the
ring sideroblasts associated with myelodysplasia. The greatest likelihood
is that a plethora of defects exists, reflecting the heterogeneous nature
of myelodysplasia and its associated sideroblastic anemia.

Gattermann and colleagues described at least two point mutations in
mitochondrial DNA of patients with acquired sideroblastic anemia (Gattermann
et
al, 1997). One mutation was a T to C change at nucleotide 6742 of the
mitochondrial genome. The affected gene encodes cytochrome c oxidase subunit
1. The mutation produced an aberrant protein in which a threonine residue
replaced isoleucine at residue 280. The other mutation also involved a
T to C transition, this time at nucleotide 6721 of the mitochondrial genome.
The defect again altered cytochrome c oxidase subunit 1, resulting in a
change from methionine to threonine at residue 273.

The fact that mitochondria from other tissues of these patients showed
no abnormality was consistent with an acquired defect solely involving
the hematopoietic stem cells. Further investigation proved these mutations
to be heteroplasmic. That is, the affected cells have a mixture of normal
and mutant mitochondria.

Technical limitations precluded analysis of mitochondrial function in
samples derived directly from the patients. The investigators therefore
performed experiments using an artificial construct (Broker et al,
1998). They used Rh-0 cells from the immortalized cell line 143B as the
starting material. Rho-0 cells are selected to have no functioning mitochondria
through repetitive treatments with ethidium bromide followed by low-level
ultraviolet irradiation. The investigators fused platelets from the affected
patients with the Rho-0 143B cells, which then contained patient mitochondria
(platelets were used because they contain mitochondria but no nuclear DNA).
The heteroplasmic nature of the mitochodrial defect in the patients allowed
the investigators to select clones that contained only mutant mitochondria
and clones that contained only wild-type mitochondria.

No difference in the growth characteristics existed between the normal
and mutant reconstituted Rho-0 143B cells. Differences in respiration between
the reconstituted wild-type and mutant mitochondria cells were modest at
best. Equivocal results and a convoluted testing system prevent a firm
statement about mitochondrial function and its relationship to the sideroblastic
anemias. The same group of investigators reported another mitochondrial
DNA mutation that affected one of the mitochondrial transfer RNAs (Gattermann
et
al, 1996). The functional consequence, if any, of the mutation is unknown.

Drug- and Toxin-induced sideroblastic anemias

Drugs and toxins are important causes of sideroblastic anemias. Table 2
lists some of the agents known to produce sideroblastic anemia. The compounds
most commonly implicated do so by inhibiting steps in the heme biosynthetic
pathway. Usually the sideroblastic anemia corrects with elimination of
the offending agent. Ethanol is the most frequent cause of toxin-induced
sideroblastic anemia (Larkin et al, 1984; Lindenbaum et al,
1980). The complication is uncommon, but the use (and misuse) of the agent
is widespread. Ethanol probably causes sideroblastic anemia by two mechanisms:
direct antagonism to pyridoxal phosphate and/or associated dietary deficiency
of this compound (McColl et al, 1980). The bone marrow changes associated
with ethanol toxicity include vacuoles in the normoblasts in addition to
sideroblasts. Interestingly, chlorampenicol commonly produces vacuoles
in the normoblasts and likewise can induce sideroblastic anemia. Vacuoles
in normoblasts and myeloid precursors are prominent in the sideroblastic
anemia of Pearson's syndrome. Vacuoles most likely result from injury or
stress to the cells.

Table 2. Drug or Toxin Induced Sideroblastic Anemia

Drugs

ethanol

isoniazid (INH)

cycloserine

chloramphenicol

busulfan

Copper Chelators

penicillamine

triethylene tetramine dihydrochloride (Trientene)

Toxins

lead

zinc

auto-antibodies

Isoniazid frequently causes sideroblastic anemia (Sharp et al,
1990). Pyridoxine prophylaxis is part of treatment regimens involving the
drug in order to prevent this complication. Isoniazid-induced sideroblastic
anemia caused a number of deaths before investigators made the connection
between the drug and the severe sideroblastic anemia. The drug markedly
inhibits ALAS activity (Pasanen, 1981).

Chloramphenicol inhibits mRNA translation by the 70S ribosomes of prokaryotes.
The drug does not affect 80S eukaryotic ribosomes. The majority of mitochondrial
proteins are encoded by nuclear DNA and are imported into the organelles
from their site of synthesis in the cytosol. Mitochondria retain the capacity
to translate a few proteins encoded by the mitochondrial genome on indogenous
ribosomes. True to its prokaryotic heritage, mitochondrial ribosomes are
similar to those of bacteria, meaning that chloramphenicol inhibits protein
synthesis by these ribosomes. Chloramphenicol-induced sideroblastic anemia
is believed to result from this inhibition. Animal studies have documented
diminished ALAS and ferrochelatase activity in cases of sideroblastic anemia
secondary to chloramphenicol intoxication (Rosenberg et al, 1974).

More recently, Leiter and co-workers examined the effects of chloramphenicol
on cellular iron metabolism using the human erythroleukemia cell line,
K562 as a model system (Leiter et al, 1999). As expected, chloramphenicol
inhibited oxidative metabolism, reduced the activity of cytochrome c oxidase,
and lowered the ATP content of the cells. Chloramphenicol also markedly
reduced the production of ferritin and the transferrin receptor by the
cells. The effect was surprising since the eukaryotic ribosomes in the
cytosol are sites of synthesis of these two iron-related proteins. Chloramphenicol
did not inhibit the synthesis other cytoplasmic or membrane proteins of
the cell. The investigators concluded that the previously unsuspected link
between mitochondrial function and cellular iron metabolism might contribute
to the microcytic, hypochromic anemia that often develops even in the absence
of sideroblastic changes in the bone marrow.

Lead intoxication is a particularly insidious cause of sideroblastic
anemia (Goyer, 1993). Iron deficiency enhances lead absorption, meaning
that the two conditions are often concomitant (Gerson, 1990). Children
with lead intoxication more commonly develop sideroblastic anemia than
do adults (Balestra, 1991). The basis of the discrepancy is not clear.
The diminishing use of lead-based paints has reduced but not eliminated
lead as a problem for children. Soil in some regions of the world has an
intrinsically high lead content, and the element attains high levels in
many plants used for food. Many countries have banned gasoline supplemented
with lead. Unfortunately, the ban is not worldwide. Many less affluent
countries continue to use gasoline containing lead since it provides more
energy per litre and is less expensive than the more highly refined gasoline.
Engine emissions deposit lead both in soil and drinking water.

Occasionally, toxic insults to the bone marrow produce sideroblastic
anemia. In one report, a patient with chronic myelogeous leukemia developed
sideroblastic anemia when placed on busulfan therapy (Fernandez, et al.,
1988). The sideroblastic anemia remitted with cessation of the busulfan
treatment. One child developed sideroblastic anemia due to antibodies that
suppressed erythroid maturation (Ritchey, et al., 1979). IgG purified from
the patient's plasma suppressed in vitro growth of erythroid precursors.
After no response to a course of steroid therapy, the child achieved a
complete remission with a round of immunosuppressive therapy. Other instances
of anemia secondary to immunosuppression of erythropoiesis, such as pure
red cell aplasia, do not have sideroblastic characteristics. The apparently
unique nature of the autoantibiody in the described case remains a mystery.

Finally, overdose of chelators such as penicillamine or triethylene
tetramine dihydrochloride (Trientene or TTH) used in treatment of Wilsonís
disease can produce sideroblastic anemia. Excessive chelation produces
a relative copper deficiency. Copper catalyzes the last step in heme biosynthesis,
insertion of iron into protoporphyrin IX. Zinc intoxication has led to
sideroblastic anemia in patients using excessive amounts of vitamin supplementation
(Porea, et al., 2000). Excessive zinc produces widespread metabolic disturbances,
including depressed levels of serum copper. The latter abnormality could
be the proximate defect in zinc-induced sideroblastic anemia. Fortunately,
the condition reverses with cessation of zinc ingestion.

Nutritional factors

Nutrients involved in the biosynthesis of heme include pyridoxine and copper,
among others. The role of pyridoxal phosphate, a metabolite of pyridoxine,
has been mentioned. Primary pyridoxine deficiency, usually secondary to
malnutrition, occasionally causes sideroblastic anemia. However other manifestations,
such as peripheral neuropathy and dermatitis dominate the clinical picture.

CLINICAL MANIFESTATIONS AND DIAGNOSIS

Sideroblastic anemias tend to be moderate to severe conditions with hemoglobin
levels ranging usually from 4 to 10g/dl. Patients have the usual symptoms
of anemia including fatigue, decreased tolerance to physical activity,
and dizziness. Other symptoms and signs not related to anemia can also
be present and may point to a cause of the condition (e.g. alcoholism).

The history should include detailed questions concerning possible toxin
or drug exposures, since these are reversible conditions. A detailed family
history looking for anemia, particularly in male relatives, is important.
Most hereditary sideroblastic anemias present in childhood. However, we
are now recognizing milder cases of hereditary sideroblastic anemia whose
symptoms do not draw attention until adulthood. Severe forms of most diseases
are usually the first described. Over time, a broader clinical spectrum
with mild or formes furstes of the conditions becomes apparent. No pathognomonic
physical finding exists for sideroblastic anemia.

The bone marrow picture in sideroblastic anemia was described earlier.
The blood smear sometimes reveals basophilic stippling, hypochromia and
microcytosis, although normocytosis and macrocytosis are possible, particularly
in myelodysplastic syndromes. A dimorphic red cell population is characteristic
of female carriers of the hereditary conditions.

Iron deficiency can coexist with sideroblastic anemia. This scenario
is particularly common in patients with myleodysplasia who can have chronic
gastrointestinal bleeding due to platelet problems. Iron deficiency can
mask sideroblastic anemia. Sideroblastic anemia remains in the differential
diagnosis of patients with iron deficiency and anemia that is refractory
to iron replacement. A repeat bone marrow following iron replacement can
show ring sideroblasts not seen in the initial sample.

Iron overload is more common than deficiency, however, even in patients
without a significant blood product transfusion history. The exact cause
of iron overload in sideroblastic anemia patients is unclear. Coexisting
hemochromatosis gene mutations do not appear to be responsible (Beris et
al, 1999). Ineffective erythropoiesis, as occurs with thalassaemia,
can accelerate iron absorption from the gut. The ineffective erythropoiesis
associated with sideroblastic anemia is much milder and does not completely
explain the iron overload. Iron overload is a particular problem for patients
with pure sideroblastic anemia. They are less likely to fall victim to
the complications produced by myelodysplasia. Consequently, they can live
long enough so that problems related to iron
overload, including congestive heart failure and cirrhosis, become
life-threatening issues.

TREATMENT

The first step in the treatment of sideroblastic anemia is to rule out
reversible causes including alcohol or other drug toxicity, as well as
exposure to toxins. The treatment of sideroblastic anemia is largely supportive,
consisting primarily of blood transfusions to maintain an acceptable hemeoglobin
level. A trial of pyridoxine at pharmacological doses (500mg per Os
daily) is a reasonable intervention since it has few drawbacks and is an
enormous benefit in those cases where it works (Murakami et al,
1991). A complete response to pyridoxine generally occurs in cases resulting
from ethanol abuse or the use of pyridoxine antagonists. Discontinuation
of the offending agent hastens recovery. Some patients with hereditary,
X-linked sideroblastic anemia also respond to pyridoxine. Improvement with
pyridoxine is rare for sideroblastic anemias of other etiologies.

After obtaining baseline parameters (red cell indices, iron studies),
the initial dose of pyridoxine should be 100-200mg daily by mouth with
a gradual escalation to a daily dose of 500mg. Folic acid supplementation
compensates for possible increased erythropoiesis, should the pyridoxine
work. A reticulocytosis occurs within 2 weeks in responsive cases, followed
by a progressive increase in the hemoglobin level over the next several
months. The maintenance dose of pyridoxine is that which maintains a steady-state
hemoglobin level. Microcytosis often persists, but is of no clinical significance.

Except in toxin-induced cases, pyridoxine treatment is usually indefinite.
Patient compliance or drug side effects can limit the treatment regimen.
Fortunately, side effects are rare with daily doses of less than 500mg.
Some patients on doses in excess of 1000mg daily have developed a reversible
peripheral neuropathy. In responsive patients, anemia recurs with discontinuation
of the pyridoxine.

Many patients with sideroblastic anemia require chronic transfusion
to maintain acceptable hemoglobin levels. Patient symptoms rather than
an absolute hemoglobin level or hematocrit
should guide transfusion. This will limit the adverse consequences of transfusion,
which include transmission of infections, allo-immunization and secondary
hemeochromatosis.

Even in patients with no meaningful transfusion history, some authorities
advocate yearly monitoring of the ferritin level and transferrin saturation.
Iron chelation with desferrioxamine
is the standard treatment for transfusional hemeochromatosis. Occasionally,
patients with a modest anemia (e.g., hemeoglobin=10 g/dL) who are not transfusion-dependent
will tolerate small-volume phlebotomies to remove iron. In some cases,
the anemia improves with removal of excess iron (Hines, 1976; French et
al, 1976). This could reflect a reduction in mitochondrial injury by
iron-mediated reactive oxygen species. This is pure speculation, however,
and the scenario is distinctly unusual.

Anecdotal reports and small case series describe allogeneic bone marrow
or stem cell transplantation for sideroblastic anemia (Gonzalez et al,
2000; Urban et al, 1992). The obvious advantage is the possibility
of cure, as has occurred in patients with ß-thalassemia.
Possible cure must be balanced against transplant complications, particularly
in older people. Families with severe forms of hereditary sideroblastic
anemia should receive genetic counseling.

CONCLUSION

Sideroblastic anemias vary in etiology and pathophysiology. The
common thread in these disorders is distinct biochemical abnormalities
affecting heme biosynthesis. Recent discoveries improved our understanding
of the interplay between mitochondrial function, heme biosynthesis, and
cellular iron metabolism. This new knowledge likely will point the way
to improved therapeutic modalities.

Cotter, PD, Rucknagel, DL, Bishop, DF. 1994. X-linked sideroblastic anemia:
identification of the mutation in the erythroid-specific delta-aminolevulinate
synthase gene (ALAS2) in the original family described by Cooley. Blood
84: 3915-3924.